Intermittent Thump Issue with with my new ST35 board

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Intermittent Thump Issue with with my new ST35 board

Postby HiFiGuy » Fri Apr 23, 2010 2:09 pm

I just built an ST35 board. I am using an ANTEK Power Transformer, and the Triode OPT's. I am getting an intermittent low thump when my amp is on. It sounds as if the power is dropping out and coming back. I have not verified this but thats what it sounds like. Has any one else had this issue, how do I fix it? I am using Telefunken Smooth Plate 12AX7, GE 1958 5814 (12AU7) and a quad of reissue Mullard EL84 Tubes. Any help would be greatly appreciated.
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Postby dcriner » Fri Apr 23, 2010 4:17 pm

Just one channel or both?
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Problem

Postby HiFiGuy » Fri Apr 23, 2010 5:26 pm

It happens in both channels. The only common is the power supply. I keep looking at the blue glow in the tube to see if it dims when the drop out occurs. But I always miss the occurrence. There were two mods done to the original design. One I used 0.22 uF Coupling capacitors, and I installed a snubber network across the diodes (0.01uf @ 1000V in series with 470 ohm resistor). I was going to snip those out and see if that was the problem.
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Re: Problem

Postby dcriner » Fri Apr 23, 2010 6:22 pm

HiFiGuy wrote:I keep looking at the blue glow in the tube to see if it dims when the drop out occurs.
Which tube? The rectifier?

That could indicate a gassy tube. Try a different tube.
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Tube

Postby HiFiGuy » Fri Apr 23, 2010 7:06 pm

Inside the output tubes, all 4 tubes have this blue inside the plate structure. This blue glow is normal. It is only observable through the holes in the plate. Could the snubber network across the rectifier diodes be causing this drop out?
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Postby antiquekid3 » Fri Apr 23, 2010 8:01 pm

What have you set the plate current per tube to? I know I don't see a blue glow at all from my output tubes. They are set at 40mA per tube.

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Postby Ty_Bower » Fri Apr 23, 2010 8:21 pm

antiquekid3 wrote:I know I don't see a blue glow at all from my output tubes.


It depends on the tube. Russian (Reflektor) tubes generally tend to produce a distinctive blue glow. Some tubes of other origin may as well. You might see a pale shade of blue on the insides of the plates (seen only through the plate holes, if present). You might see a darker shade of blue on the inside surface of the glass. The intensity of the glow depends more on voltage rather than current. My DIY ST35 typically runs over 410 volts B+. I get a fair amount of blue glow from my Russian 6pi14pi tubes, but sometimes you really have to be looking to see it.

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update

Postby HiFiGuy » Fri Apr 23, 2010 10:51 pm

As per manual instructions I have all the tubes set to 35 mA current draw. I made this adjustment after 3 hours of warm up time so that all voltages and temperatures stabilized. These are Mullard Reissue tubes and are made in Russia. I have used many different types of EL84 tubes and all versions with holes in the plates I have noticed the light blue glow. The one tube I have used with no holes is the RCA. Although these have no holes the light blue can be made out through the top or bottom mica. This blue glow can only be seen in a completely dark room and only faintly. This problem I am having with my amp is very annoying its as if the volume is being turned down then back up. Other than this issue the amp sounds great. Thank You for all your comments keep em coming!!
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Solution

Postby HiFiGuy » Sat Apr 24, 2010 9:10 pm

Its official the ST35 board DOES NOT like snubber networks on the Rectifier Diodes. I snipped em out and the problem is gone. No drop out after 1 hour of use. If it comes back I will post in here again. Thanks
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Postby dcgillespie » Sun Apr 25, 2010 8:47 am

There may be a good chance that the problem could be solved by using the standard coupling cap value as well. Since you are using over double the original value, the low frequency stability of the design may be upset enough for a transient to trigger an oscillation that momentarily saturates the output stage at low frequency, causing the drop out. The length of the event would be determined by the time constants of the coupling caps and the cathode bypass cap. This would be particularly true if the original level of NFB is used, and would explain the intermittent nature of the event.

If this is happening, it is akin to some of the early Williamson amplifiers, where in the low frequency stability was so bad, you could watch the woofer routinely "breathe" in and out without any signal applied at all.

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Postby Vince » Sun Apr 25, 2010 9:13 am

Great post Dave!! Vince
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Update

Postby HiFiGuy » Tue Apr 27, 2010 10:31 pm

I took the speaker covers off of my speakers and, sure enough the speakers are "breathing". I am going to replace the coupling capacitors the next chance I get. Thanks for your help!!
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Postby dcgillespie » Wed Apr 28, 2010 8:59 am

Hifi --

It's good you checked for that condition as when it occurs like that, it robs power, causes wild IM distortion, and is not very kind to the output tubes either. At the very least, it is an excellent example of how stability in a NFB amplifier can be upset by simple seemingly inocent changes to the design. In your case, I don't know what output transformer you used, but it also plays a huge roll in both the low and high frequency stability issue. Hafler's original transformers were excellent in this regard, and while there are other good transformers out there, changing them into the circuit would still very likely require changes in the stability networks to maintain the stability of the original design. It may very well be that (possibly) using other "similar" transformers along with the change in coupling cap values is what caused the instability -- where as either change by itself may not have been a problem.

Hafler was king of the stability issue when his Dynaco amps first came along -- at a time when some famous Heath and even an Eico or two were rather famous for burning out tweeters due to their instability, or had the breathing woofers as I previously mentioned. A properly running Dynaco ST-35 is stable into any kind of load -- even capacitive only loads.

Negative Feedback is simple in concept, but very complex to apply correctly. It is at the root of your issue, as if you disconnected the NFB loop, the breathing would go away. But then you would also lose all the benefits it produces of low distortion, high damping, and improved frequency response. In fact, it is these very kinds of situations that cause so many people to dismiss NFB as a bad thing, which is a shame. Applied correctly, it produces so many benefits, and few concerns. But so many who either don't understand the issues involved, or tried to make it work and couldn't, just dismiss it all together, and give it a bad name that in concept it doesn't deserve.

Installing the original coupling cap values is the first step. Then, regardless of what test equipment you have available (although it's extremely helpful), there are some simple tests you can do to get an idea of how stable your amplifier really is. Keep us posted!

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Update

Postby HiFiGuy » Wed Apr 28, 2010 4:52 pm

I started the amplifier with all Antek Iron. I built a custom chassis and the 3 toroid transformers looked really cool. When all was finished I hooked it up to my scope and at full power (5 Watts) the bandwidth was 20 - 8 KHz. If I tried any higher power the waveform would get badly distorted. I ended up using the toroid power transformer and the OPT's from the Triode site. The triode transformers at full power (17 Watts) had a bandwidth from 17 Hz - 43 KHz, BETTER THAN THE TOROID!!! They sound much better and have a lot more headroom. Other than this Breating Woofer issue everything is great. After I get those new coupling capacitors in I will start doing some tube rolling and see which combination I like. I have lots of foreign and domestic tubes here to try (12AX7 and 12AU7 Variants). I was thinking of trying to build a stereo amp (Point to Point on Turrets) either single ended or push pull using some Amperex 6227 tubes. Thanks again for your help.
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Postby dcgillespie » Wed Apr 28, 2010 6:00 pm

That sounds like a great amp you've built. I'm particularly happy to hear that the Dynaclone transformers perform so well. I have an original ST-35 I use as a reference for what the original sounds like, but am glad to know that the triode transformers are so well made. It will be interesting if going back to the original value coupling cap solves the LF stability problem. Did you say at one point that you otherwise followed the original design to a tee? Did that include the input coupling cap? Sometimes I've seen that cause problems when it is omitted.

Do you have the specs on the Antek transformers? I'd be interested to know what they are.

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